Preheat the oven to 200 degrees Celsius (392 degrees Fahrenheit) (fan).

Butter 6 molds with butter.

Melt the butter and chocolate in a bowl over a water bath. While it melts, whisk the eggs and sugar until light and frothy, then add the flour and salt and whisk well together.

When the butter and chocolate have melted, stir until a uniform mixture forms, then gently fold it into the egg mixture.

Divide the batter into the buttered molds (for better control, you can use a piping bag or a pitcher with a spout).

Bake the cakes in the middle of a preheated oven for 6 minutes.

Let the cakes cool for 5 minutes before serving and dust with powdered sugar using a sieve.